1. Field of the Invention
The present invention relates to an active matrix substrate to be used in a display device, such as a liquid crystal display device or EL (electroluminescence) display device. More particularly, it relates to an active matrix substrate suited for use in large-size liquid crystal television or a like liquid crystal display device equipped with a large-size liquid crystal display panel.
2. Description of the Related Art
Active matrix substrates are widely used in active matrix drive type display devices such as liquid crystal display devices and electroluminescence (EL) display devices. In such active matrix drive type display devices, active elements are disposed to form a matrix on respective independent pixel electrodes, and screen displaying is made by the active matrix drive method according to which the pixel electrodes are selectively driven by these active elements. Generally used as the active elements for selective driving of pixel electrodes are TFT (thin film transistors) elements, MIM (metal-insulator-metal) elements, MOS transistor elements, diodes, and varistors, among others, and screen displaying is made by switching the voltage applied to each pixel electrode and the corresponding opposing electrode to thereby optically modulate the display medium, such as a liquid crystal layer, EL display layer or plasma radiation layer, between both the electrodes. Such active matrix drive method enables high contrast displaying and is in practical use in liquid crystal televisions, personal computer terminal display devices, and so forth.
As the conventional active matrix substrate for use in a liquid crystal display device using such active matrix drive method, one in which the drain electrode of a TFT element is connected to the corresponding pixel electrode via a drain extraction line and a contact hole has been disclosed (cf. e.g. Japanese Kokai Publication H10-20298, pages 3 and 6, drawing 1). In this conventional display device, only one drain extraction line is formed for each pixel on the active matrix substrate and, in case of breaking of this one drain extraction line, the normal display of the corresponding pixel is not made any longer, which leads to the occurrence of an operation failure called pixel defect; the result is a decrease in the yield of liquid display devices. This is explained referring to the drawings. In the conventional active matrix substrate for a liquid display device, as shown in FIG. 13-1, breaking 22 of one drain extraction line, as shown in FIG. 13-2, results in inhibition of a data signal 21 from arriving at a pixel electrode for transmission from a source bus line 5 via a drain electrode 1 and the drain extraction line 2. As a result, a pixel defect occurs, and this lowers the display quality of the liquid display device and causes a decrease in yield. As the cause of drain extraction line breaking, there may be mentioned, among others, a photo resist pattern defect in wiring pattern formation, and a defect in layer deposition in drain extraction line layer formation by sputtering, for instance.
As a technology of preventing the occurrence of pixel defects, a liquid crystal display device has been disclosed in which each pixel is provided with a plurality of thin film transistors (cf. e.g. Japanese Kokai Publication H07-199221, pages 3 and 6, drawing 1 and Japanese Kokai Publication 2002-350901, pages 13 and 20, drawing 9). However, to provide each pixel with a plurality of thin film transistors causes a decrease in aperture ratio and an increase in production cost; in these respects, there is room for improvement.
Further, a liquid crystal display device or the like in which an interconnection line (bridge) is disposed between two neighboring pixel electrodes to cope with the occurrence of a pixel defect by connecting the electrode of the defective pixel to the electrode of the neighboring normal pixel by means of that interconnection line has been disclosed (cf. e.g. Japanese Kokai Publication H02-135320, pages 3 and 4, drawing 1 and Japanese Kokai Publication H08-328035, pages 3 and 5, drawing 1). According to this technology, however, the interconnection line is disposed in a manner stepping across the gate line, so that the gray scale characteristics are deteriorated due to the increase in coupling capacity; in this respect, there is room for improvement.
In recent years, the liquid display panels in liquid crystal televisions and so forth have become larger and larger in size and, accordingly, the number of pixels has been increased; as a result, there is a tendency for the number of pixel defects to increase. The size of pixels is also increasing with the increase in size of the screen and, therefore, even when an abnormal bright spot, if discovered in the manufacturing process, is converted to a dark spot, as is conventional in the art, users will be able to readily recognize, by the eye, not only the luminescent spot but also the dark spot as a pixel defect. Under such circumstances, a novel technology has been demanded for improving the display quality by effectively inhibiting pixel defects from occurring and thereby improving the yield.